Review of Cardiac Review of Cardiac Structure and Structure and FunctionFunction

LocationLocation

Heart lies in the mediastinum behind Heart lies in the mediastinum behind the sternumthe sternum

1/3 of bulk of the heart lies to the 1/3 of bulk of the heart lies to the right of the midline; 2/3 lies to the leftright of the midline; 2/3 lies to the left

Apex lies between the 4Apex lies between the 4thth & 5 & 5thth ribs ribs when lying down and between the 5when lying down and between the 5thth and 6and 6thth ribs when sitting or standing ribs when sitting or standing

PericardiumPericardium The heart lies within the pericardiumThe heart lies within the pericardium Outer layer of the pericardium is Outer layer of the pericardium is

fibrous connective tissue – fibrous fibrous connective tissue – fibrous pericardiumpericardium

Anchors the pericardium to the great Anchors the pericardium to the great vessels, the sternum and diaphragmvessels, the sternum and diaphragm

Two inner layers of serous Two inner layers of serous membranes- parietal pericardium and membranes- parietal pericardium and visceral pericardium (epicardium)visceral pericardium (epicardium)

Between the serous membranes is Between the serous membranes is the pericardial space, filled with 15- the pericardial space, filled with 15- 50 ml of fluid to reduce friction50 ml of fluid to reduce friction

Fibrous pericardium and parietal Fibrous pericardium and parietal pericardium are called the pericardial pericardium are called the pericardial sacsac

Visceral pericardium is the Visceral pericardium is the epicardiumepicardium

Heart WallHeart Wall EpicardiumEpicardium MyocardiumMyocardium – muscular layer – muscular layer

– Thickest layerThickest layer– Attached to fibrous skeleton of the Attached to fibrous skeleton of the

heartheart– Bands of muscle arranged Bands of muscle arranged

longitudinallylongitudinally– Fibers from one side enter other sideFibers from one side enter other side– Better integration of contractionsBetter integration of contractions– Pathology of one ventricle can affect Pathology of one ventricle can affect

the otherthe other

EndocardiumEndocardium– Internal lining of squamous Internal lining of squamous

epitheliumepithelium– Continuous throughout Continuous throughout

cardiovascular systemcardiovascular system

Four chambers (two Four chambers (two pumps)pumps) Right heart acts as a volume Right heart acts as a volume

pump – through low-resistance pump – through low-resistance vessels of the pulmonary systemvessels of the pulmonary system

Left heart acts as a pressure Left heart acts as a pressure pump – through high-resistance pump – through high-resistance vessels of the systemic circulationvessels of the systemic circulation

AtriaAtria

Two upper, thin walled chambers Two upper, thin walled chambers that collect blood returning to the that collect blood returning to the heartheart

Their contraction aids in filling the Their contraction aids in filling the ventriclesventricles

Right atrium receives blood from Right atrium receives blood from superior and inferior vena cavae superior and inferior vena cavae and coronary sinusand coronary sinus

Left atrium receives blood from Left atrium receives blood from pulmonary veinspulmonary veins

VentriclesVentricles Thick walled pumping chambers of Thick walled pumping chambers of

the heartthe heart Make up about 60 % of the mass of Make up about 60 % of the mass of

the heart and receive most of the heart and receive most of coronary blood flowcoronary blood flow

Blood from right ventricle enters the Blood from right ventricle enters the pulmonary trunkpulmonary trunk

Blood from left ventricle enters the Blood from left ventricle enters the aortaaorta

Path of Blood FlowPath of Blood Flow

Vena cavae Vena cavae →→ RA RA→→ RV RV→→

Pulmonary trunkPulmonary trunk→→ Lungs Lungs→→

Pulmonary veinsPulmonary veins→→ LA LA→→ LV LV→→

Aorta Aorta → → Systemic Systemic circulationcirculation→→

Vena cavae Vena cavae

ValvesValves

Made of connective tissue covered Made of connective tissue covered by endotheliumby endothelium

Prevent back flow of bloodPrevent back flow of blood Open and close Open and close passively passively in in

response to ventricular contractionresponse to ventricular contraction Anchored to the annuli fibrosi Anchored to the annuli fibrosi

cordis to prevent dilation during cordis to prevent dilation during contraction of the heartcontraction of the heart

Atrioventricular valvesAtrioventricular valves

Lie between the atria and Lie between the atria and ventriclesventricles

Tricuspid valve on the rightTricuspid valve on the right Bicuspid or mitral valve on leftBicuspid or mitral valve on left Anchored against high pressure Anchored against high pressure

by the chordae tendineae and by the chordae tendineae and papillary musclespapillary muscles

Semilunar ValvesSemilunar Valves

Lie between ventricles and great Lie between ventricles and great vesselsvessels

Pulmonary on right sidePulmonary on right side Aortic on left sideAortic on left side

Cardiac CycleCardiac Cycle

Sequence of events that compose Sequence of events that compose the repeating pumping action of the the repeating pumping action of the heartheart

Typically, Typically, systolesystole refers to refers to ventricular contraction and ventricular contraction and diastolediastole refers to ventricular relaxationrefers to ventricular relaxation

If referring to atria, specify atrial If referring to atria, specify atrial systole, etc.systole, etc.

ReviewReview

Coronary arteries and cardiac Coronary arteries and cardiac veins – note that it is the right veins – note that it is the right coronary artery that supplies coronary artery that supplies blood to the SA and AV nodesblood to the SA and AV nodes

ReviewReview

Pressures in chambers: look over Pressures in chambers: look over the next figure - note especially the next figure - note especially the pressures in the right atrium – the pressures in the right atrium – central venous pressure, and the central venous pressure, and the pressures in the aorta – systemic pressures in the aorta – systemic blood pressureblood pressure

Cardiac muscle - Cardiac muscle - MyocytesMyocytes Contractile fibersContractile fibers Fibers are branchedFibers are branched Intercalated discs - containIntercalated discs - contain

desmosomes desmosomes and and gap junctionsgap junctions Gap junctions connect cytoplasm of Gap junctions connect cytoplasm of

adjoining cells so that the action adjoining cells so that the action potentials pass from one cell to the potentials pass from one cell to the nextnext

Cells contract as if one cell - syncytiumCells contract as if one cell - syncytium

Muscle contractionMuscle contraction

Striated muscle – contains actin and myosinStriated muscle – contains actin and myosin Action potential stimulates release of CaAction potential stimulates release of Ca++++

CaCa++++ binds to troponin binds to troponin Troponin moves the tropomyosin exposing Troponin moves the tropomyosin exposing

myosin binding sites on the actinmyosin binding sites on the actin Myosin pulls on the actin, filaments slide Myosin pulls on the actin, filaments slide

past each other and muscle shortenspast each other and muscle shortens

Remember, that unlike skeletal Remember, that unlike skeletal muscle, much of the Camuscle, much of the Ca++++ used in used in cardiac muscle contraction comes cardiac muscle contraction comes from the extracellular fluid.from the extracellular fluid.

Conduction System Conduction System Conduction system cells are Conduction system cells are

specialized myocardial fibersspecialized myocardial fibers Heart has autorhythmicity- beats Heart has autorhythmicity- beats

spontaneously at about 100 beats/minspontaneously at about 100 beats/min Impulse begins at sinoatrial (SA) node Impulse begins at sinoatrial (SA) node

or pacemaker – sinus rhythmor pacemaker – sinus rhythm Spreads through atria via conducting Spreads through atria via conducting

myofibers to atrioventricular (AV) node myofibers to atrioventricular (AV) node in fibrous skeleton of heartin fibrous skeleton of heart

Signal pauses briefly, for ventricular Signal pauses briefly, for ventricular filling by atrial contraction.filling by atrial contraction.

Signal continues into ventricle Signal continues into ventricle through conduction pathways: through conduction pathways:

AV bundle ( bundle of His) AV bundle ( bundle of His) →→right right and left bundle branchesand left bundle branches→→ Purkinje Purkinje fibersfibers→→ ventricular contraction ventricular contraction (bottom up)(bottom up)

Sympathetic N.S. increases heart Sympathetic N.S. increases heart rate and force of contraction – rate and force of contraction – secrete norepinephrine –accelerator secrete norepinephrine –accelerator nervesnerves

Parasympathetic N.S. decrease heart Parasympathetic N.S. decrease heart rate and force of contraction through rate and force of contraction through the vagus nerve. Sends continuous the vagus nerve. Sends continuous impulses. Secretes acetylcholineimpulses. Secretes acetylcholine

Regulation of Heart Regulation of Heart RateRate

Electrocardiogram ECGElectrocardiogram ECG

Measures electrical activity of the Measures electrical activity of the heartheart

P wave – represents atrial contraction P wave – represents atrial contraction (depolarization)(depolarization)

QRS complex – represents ventricular QRS complex – represents ventricular contraction (depolarization)contraction (depolarization)

T wave – represents ventricular T wave – represents ventricular relaxation (repolarization)relaxation (repolarization)

Cardiac OutputCardiac Output

Cardiac Output:Cardiac Output: SV (ml/beat) X HR (beats/min) = SV (ml/beat) X HR (beats/min) =

CO(ml/min.) CO(ml/min.) 70 ml X 80 = 5600 ml 70 ml X 80 = 5600 ml

/min. or /min. or 5.6 5.6 liters L/minliters L/min

Factors affecting Factors affecting cardiac performancecardiac performance

PreloadPreload: pressure generated at the end : pressure generated at the end of diastole; depends on both heart and of diastole; depends on both heart and vascular system –the amount of filling of vascular system –the amount of filling of the ventricle during relaxationthe ventricle during relaxation

AfterloadAfterload: resistance to ejection during : resistance to ejection during systole; depends on both heart and systole; depends on both heart and vascular system - the force that opposes vascular system - the force that opposes ejection of blood from the heart; for the ejection of blood from the heart; for the LV, this is the aortic systolic pressureLV, this is the aortic systolic pressure

Heart rateHeart rate: a intrinsic characteristic : a intrinsic characteristic of the heart tissue that is influenced of the heart tissue that is influenced by nervous and endocrine systemsby nervous and endocrine systems

Myocardial contractilityMyocardial contractility: the : the ability of the heart muscle to ability of the heart muscle to contract, the force of contraction - contract, the force of contraction - another cardiac tissue characteristic another cardiac tissue characteristic that is influenced by nervous and that is influenced by nervous and endocrine systemsendocrine systems

(Frank-) Starling Law(Frank-) Starling Law

Within limits, the greater the Within limits, the greater the stretching of the muscle fibers stretching of the muscle fibers (preload), the greater the force of (preload), the greater the force of contraction.contraction.

The extra force of contraction is The extra force of contraction is necessary to pump the increased necessary to pump the increased volume of blood from the ventricle.volume of blood from the ventricle.

Cardiac output increasesCardiac output increases

Neural reflexesNeural reflexes

Bainbridge reflex – Bainbridge reflex – increasedincreased heart rate due to heart rate due to increased increased right atrialright atrial pressure pressure

Increased pressure Increased pressure in arteriesin arteries stimulates a baroreceptor reflex stimulates a baroreceptor reflex that that decreasesdecreases heart rate. heart rate.